1. Field of the Invention
The present invention relates to a co-extruded product for structural applications having a metallic foam core with a skin structure, wherein the skin structure may also be metallic and methods of making the same.
2. Prior Art
Aluminum and aluminum alloy extrusions are used for various applications in several industries such as in building, construction, transportation, and infrastructure. Typical examples of such applications are window frames, door frames, automotive frames, bus frames, aircraft frames, bridge frames, frames for ships and boats, light poles, and the like. Certain physical and mechanical characteristics of aluminum alloy extrusions preclude them from being considered as suitable materials in certain applications.
For example, in building and construction industries, aluminum alloy extrusions (also termed profiles) are often restricted to certain sections of the buildings such as top stories due to their low load bearing capacities compared to other materials such as steel. One way to improve load bearing capacity is to increase the wall thickness of the profiles, but that increases the cost due to the greater quantity of aluminum used in the profiles.
The thermal characteristics of aluminum alloy products affects their use in extrusions. In particular the thermal conductivity and thermal expansion of aluminum alloy profiles are high compared to other competing materials such as steel, wood and plastic. While in many instances this may be desirable, in a large majority of applications this is undesirable and thus some complex designs are required for the aluminum alloy products thus leading to higher costs. For example, window frames for buildings are typically made from at least two profiles, an inside profile and an outside profile, with an insulating layer of wood or plastic in between to prevent heat transfer between the outside and inside of a building. Normally, the quantity of profiles for a window frame is greater than two due to a combination of mechanical, thermal and manufacturing requirements. Thus, it is not unusual to have a set of four or five different profiles, which further increases the cost.
In the transportation industry, extruded thin walled aluminum alloy profiles are currently being explored as framing materials for auto, truck, and bus bodies. While various features of aluminum alloy profiles are attractive for these applications (e.g. high stiffness to weight ratio, higher resistance to corrosion, and manufacturability), certain other aspects of aluminum alloy profiles (e.g. sound transmission) make them undesirable. Sound generally travels faster through aluminum than steel and plastic. In addition, vehicle frames made of hollow or semi-hollow aluminum profiles behave like sound pipes and readily transmit noise and vibration to other locations in the vehicle. Sound and vibration may be reduced using thicker profiles, but at a higher cost of materials.
Another specific aspect of aluminum alloy profiles is their energy absorbing characteristics in the event of collision with another vehicle or stationary objects like light poles, side barrier, etc. Due to the limited plastic deforming capacity of alloyed aluminum extrusions compared to steels, more often, the profiles break into pieces during collision.
Many of these problems with aluminum extrusions may be overcome by increasing the thickness of the extrusions, but that increases their cost. Alternatively, extrusions may be filled with metallic foam such as aluminum foam or magnesium foam, which may provide added load bearing capacity, minimize the quantity of profiles for framing, provide sound dampening, reduce vibration and increase energy absorbing capacity without increasing the cost substantially. Extruded infrastructure products such as light poles, highway crash barriers, and the like may also be filled with cellular materials to improve their energy absorbing capacity. Foam filled extruded light poles may exhibit reduced vibrations during heavy winds. Foam filled highway crash barriers may provide improved energy absorbing capacity compared to unfilled extrusions. Polymeric foams are easily fit into a profile but present a fire hazard. Metallic foams reduce the risk from fire, but are more difficult to produce within an extrusion.
Thus, composite materials containing thin metallic skin, filled with cellular materials such as metallic foams are sought after in various fields such as building, construction, transportation, and infrastructures due to the unique combinations of properties such as light weight, high stiffness, high load bearing capacity, high energy absorption capacity, noise damping capacity and fire resistance.
Accordingly, many attempts are being made to produce these types of composite materials. One direction under consideration involves inserting a foam core into a preformed extrusion shell. The foam core may be machined out of a standard foam structure (such as a cylinder that may be manufactured by inexpensive, traditional powder metallurgical or liquid metallurgical methods) and fit into the shell. A disadvantage of this method is that there is no bonding between the shell and the foam core material, and as a result, improvements to the performance are limited. Attempts to create a bond between the foam core and the shell to improve the performance have used traditional adhesive bonding technologies, but at a substantial extra expense.
It is known in the field of casting to manufacture a composite material containing a foam core and a solid shell by casting a shell over a foam core using standard casting operations of sand casting or die casting. While this method offers a way to develop metallic bonding between the shell and the core, the process is restricted to relatively small components. In addition, the surface quality of the shell made by this method is inferior compared to an extruded shell, and the method is restricted to very few alloys that can be cast at temperatures below the temperature at which the core remains solid.
In another related field, simple composite panels are made by first cladding a sheet of aluminum alloy with a sheet of a foam pre-form (not yet a foam), which consists of a low melting alloy in powder form mixed with foaming agents such as hydrides, hydroxides, and carbonates, and then heating the clad product to temperatures above that at which the core material melts and expands into a foam core due to the decomposition of the foaming agents, not unlike the traditional polymer foams. Attempts are being made to extend this method also to make extrusions by co-extruding a billet consisting of a thick shell of material which is filled with the foam pre-form. After extrusion, the co-extruded solid can be heated to temperatures, above which the foam core melts and expands to final shape as indicated above for simple panels. This method is useful only for simple shapes, results in poor surface quality and the dimensions of profiles are difficult to control. Although these processes work, they are considerably expensive due to the need for powder metallurgical pre-forms and also restrictive with respect to the type of alloys that can be used. Thus, there remains a need for a reliable and economical method to make extruded composites having a thin extruded shell containing a foam core.
This need is met by the present invention, in which an extrusion unit is integrated with a foaming unit and a liquid metal pumping unit, so that the extruded shell can be filled as it is formed with the metallic foam, thereby guaranteeing a good metallurgical bond between the shell and the core. In the present invention, the shell metal remains solid throughout the process, albeit at very high temperatures, often close to the metal solidus temperature, while the metallic foam remains in liquid state until such time that it fills the shell. The shell and core are cooled down to room temperature at such rate that the bond remains intact. Further, since the shell is filled as it is formed, the surface quality of the profile remains the same as a standard extruded profiles.